The present invention relates to a method and apparatus for producing semiconductor or metal particles, particularly particles of a semiconductor or metal having high melting point.
A typical method for producing semiconductor or metal particles is a method in which spherical particles are produced by heating and melting a solid raw material and dropping its melt into a gaseous phase. Spherical particles produced in such a method are used, for example, as spherical semiconductors to be incorporated into a solar array. As disclosed in Japanese Examined Patent Publication No. Hei 7-54855, for example, such a solar array generates photoelectromotive force by electrically connecting spherical silicon semiconductors to a metal foil matrix and applying light to the spherical semiconductors.
A specific method for producing spherical particles is a method disclosed in Japanese Laid-Open Patent Publication No. 2002-292265, for example. In this method, a semiconductor such as silicon (Si) stored in a crucible is heated and melted, and a gas such as Ar or N2 is supplied into the space over the melt of the semiconductor in the crucible, so that the melt is dropped from the nozzle provided at the bottom of the crucible into a gaseous phase by the pressure of the supplied gas. Further, U.S. Pat. No. 4,188,177 discloses a method that uses a cylindrical crucible made of quartz glass for producing Si particles. In this method, a Si melt in the crucible is pressurized by an inert gas such as He or Ar to drop the melt from the nozzle into a gaseous phase.
As a method for producing metal particles suitable for powder metallurgy, Japanese Examined Patent Publication No. Sho 60-59283 discloses a technique of jetting a metal melt stored in a crucible from the nozzle of the crucible by applying the pressure of an inert gas such as Ar, He, Xe or Kr. Further, Japanese Patent No. 2674053 discloses a method for producing fine particles of metal such as gold, using a heat-resistant crucible. In this method, a molten metal in the crucible is pressurized by an inert gas to drop the melt from the nozzle of the crucible into a gaseous phase.
According to the experiments performed by the inventors of the present invention, in any of the above-mentioned production methods of spherical particles, the speed at which the semiconductor or metal melt stored in the crucible drops from the nozzle often lowers with the passage of time, resulting in stop of the dropping of the melt even if the pressurization by the inert gas is heightened.
This has four reasons. First, when the melt is dropped from the nozzle, the bottom face of the crucible becomes wet with the melt, so that solidified semiconductor or metal adheres to and near the nozzle on the bottom face of the crucible, thereby clogging the nozzle. Second, the melt having high temperature easily reacts with the crucible material to heighten the wettability of the crucible material by the melt, and the reaction products accumulate in and near the nozzle, thereby clogging the nozzle. Third, the reaction products of the melt and the pressurization gas accumulate in and near the nozzle, thereby clogging the nozzle. Fourth, due to the exposure of the crucible to high temperatures by the heating for melting the raw material in the crucible or the heat transmission from the melt stored in the crucible, the crucible material becomes softened, and the nozzle becomes deformed upon the application of load such as the pressurization gas to the softened crucible material, resulting in closing of the nozzle.
When the flow of the melt is hindered in and near the nozzle as described above, the dropping speed of the melt gradually lowers or the dropping stops even if the melt is pressurized by a constant gas pressure. Once the nozzle is completely closed, the melt will not drop even if the gas pressure is raised to its highest possible level. This problem cannot be solved simply by regulating the gas pressure depending on the dropping speed.
Further, in the above-described production methods of spherical particles, the reaction products of the melt and the crucible material are liable to be included in the melt as impurities, so the spherical particles produced often include large amounts of impurities. When these spherical particles are used as semiconductor elements or their bases, such impurities have significant effects on the electrical characteristics of the semiconductor elements.
In order to solve these problems, it is necessary to use a crucible which has low wettability and low reactivity with respect to a melt, which is chemically stable to the melt and which has excellent thermal resistance, in combination with the selection of a pressurization gas which is unreactive to the melt. However, it is extremely difficult to find such crucible material satisfying all the requirements. For example, tangsten, molybdenum, tantalum, alumina and the like have excellent thermal resistance, but easily react with a Si melt to produce silicides. Further, carbon, which is conventionally used as the crucible material, reacts with the Si melt to produce SiC on the surface. These reaction products considerably heighten the wettability and are liable to be included in the melt. Hence, it is not appropriate to use these materials in their natural state as the crucible materials.
Of the above-mentioned prior art techniques, when quartz glass is used as the crucible material, no problems are caused by the first and second reasons because of the extremely excellent chemical stability and non-wettability of quartz glass with respect to a high temperature melt, and further, there is no concern that impurities are included into the melt. As described above, quartz glass has excellent characteristics as the crucible material, but its thermal resistance is not necessarily sufficient. Quartz glass has such properties that its viscosity lowers as the temperature becomes higher and it therefore deforms when a load is applied thereto. The occurrence of such deformation generally becomes remarkable at a temperature of around 1200° C.
Therefore, the use of a crucible made of quartz glass for producing spherical particles of a semiconductor or metal having high melting point such as Si causes such problems that the nozzle of the crucible is closed by the fourth reason so that the dropping speed of the melt lowers or the dropping stops even if the melt is pressurized by a constant gas pressure.